Underground storage of flammable liquids, typically gasoline, volatile hydrocarbon solvents, petroleum derivatives of all kinds, as well as acids, alkaline solutions, and other potentially hazardous substances causes problems since the contents of the tanks, upon leakage thereof, may contaminate the surrounding soil. This problem was graphically described before the Toxic Substances Subcommittee of the Senate Environment and Public Works Committee by Mr. Jack E. Ravan, Assistant Administrator of the Environmental Protection Agency (EPA) in testimony late November 1983. It was noted that it is estimated that millions of gallons of gasoline leak into the ground each year from tanks at service stations and other storage areas, and pose a potentially serious threat to the nation's underground water supplies. It has previously been proposed to construct double-wall tanks, and evacuate the space between the inner and outer wall. The vacuum thus generated is monitored; upon leakage, for example due to corrosion, or other defects of either one of the walls, the vacuum will collapse quite fast; a monitoring instrument will indicate the absence of vacuum between the two walls of the vessel and thus provide an indication that leakage may occur; the tank can then be emptied and repaired before leakage of the ground has occurred if one of the walls has remained intact; if both should have been punctured, leakage to the surrounding ground area can be stopped rapidly.
Various ways to modify existing tanks to detect leakages have been proposed. On tanks for heating oils, it is known to provide an inner wall separated from an outer, existing wall. The inner wall of the existing tank is first lined with sheets of aluminum having a plurality of knobs, bumps, bosses or similar projections, or corrugations formed thereon. A plastic liner, or an inner plastic foil, is then applied to the aluminum sheet, forming the inner wall of the tank. The space defined between the inner and the outer wall is then partly evacuated and the vacuum monitored. This method of modifying tanks is suitable for tanks or vessels which store products having a relatively high flame point. It is suitable for bunker oil, and even for commercial "No. 2" home heating fuel. In general, any product which has a flaming point of about 55.degree. C. or higher may be stored in tanks of this type. Liquids which are highly flammable, however, cannot be safely stored in tanks of this type. The plastic foil or liner is, for all practical purposes, an electrical insulator, and thus electrostatic charges which arise due to friction, for example, or sloshing of the liquid upon filling or withdrawal thereof cannot be rapidly dissipated or leaked to ground. Sparks may occur upon inspection of the tank, for example, or upon other handling, e.g. outside connections. Such sparks may cause explosions of explosive gases which accumulate in a tank which is empty, or which form above the liquid level of the substances within the tank and its top.
It has been proposed to modify gasoline tanks to become leakproof by introducing relatively small panels or sheet material through a manhole and to weld the sheets together to an inner wall, thus forming, practically, a second tank inside the first tank, with a space between the tanks which can be monitored for a vacuum, as described above (see, for example, Swiss Patent No. 614,417). The inner wall, being made of sheet metal, is electrically conductive, and thus there is no danger of accumulation of electrostatic charges, and hence sparking due to accumulated charges. This method of modifying existing tanks, for example of already buried tanks, while satisfactory in result is extremely expensive, requires skilled welders, and is difficult to carry out in many installations.
It has also been proposed to provide a double-wall tank having an inner wall made of glass fiber-reinforced polyester which has an electrically conductive layer made of chrome mickel fabric, or wire mesh embedded therein (see Published European Patent Application No. 00 14 491). Manufacture of such a tank by manually applying a laminate of glass fiber-reinforced polyester and a wire mesh on the soft laminate is time-consuming and hence expensive. The arrangement has a further disadvantage: It is frequenty desirable to test the integrity of the inner liner by spark inductors, and by eddy current testers.
It is particularly important in connection with polyester laminates to check the laminate for both thickness as well as porosity. If a wire mesh is located on the laminate before it has hardened or cured, it is no longer possible to make the standard tests with spark inductors and eddy current apparatus for porosity and thickness. Thus, any testing must be done by filling the tank with a liquid, which test, however, will not discover thinner spots in the inner liner, only possible leakage.
It has also been proposed--see German Published Patent Application DE-AS No. 26 20 225--to construct a vessel of cement or concrete which has a coating of polyester resin on the bottom as well as on the side walls of the tank. The polyester resin is made of a multi-layer lamination. The vertical cement walls have a first multi-layer liner applied, made of polyester. A metallic foil, for example of about 2 mm thickness, and formed with knobs, projections, bumps, bosses or corrugations, is then applied to the first multi-component layer. A second layer is then applied, on the inside of the tank, over the intermediate metal foil, the space between the layers forming the vacuum space which can be monitored to determine leakage. To provide for electrical connection and grounding of the inner, or second layer, the polyester resin thereof is made electrically conductive by adding, for example 25% of graphite powder thereto. The specific resistance of the second, or inner layer will be about 500 meg ohms. The quantity of graphite powder is so controlled that the characteristics of the second, or inner layer with respect to strength and sealing capacity, or absence of porosity is hardly affected, but some electrical conductivity is obtained. Since the path of discharge current is across the thickness of the inner layer, and thus is short, any electrical charge which might build up on the second, or inner layer can be leaked off to the embossed metal foil therebehind, and then connected to the outer, metallic tank.
Manufacture of a vessel using this arrangement is expensive and highly labor and material intensive. The polyester laminates must be applied manually, on-the-job, and painstakingly joined. The thickness of the layer, as well as the freedom from pores, cannot be checked after application since the second, or inner layer is electrically conductive. The standard tests for thickness and porosity, thus, by spark inductor and eddy current test apparatus cannot be carried out.
It has also been proposed to provide a vessel or tank made of cement or concrete with an inner wall of polyester in which the cement walls are covered with an aluminum foil so that the inner polyester can be checked for freedom from pores and to localize any possibly occurring openings by means of a spark inductor, and to check the thickness of the polyester layer or sheet by an eddy current tester (see Swiss Patent No. 517,630). The polyester layer on the aluminum foil faces the inside of the tank and, since the polyester is highly electrically insulating, it cannot leak off charges to the aluminum foil and thus accumulate electrostatic charges. Such a tank is not suitable for highly flammable liquids, which may release explosive gases and which may cause explosions upon occurrence of a spark. Tanks of this type are only suitable for substances which have a very high flame point and cannot be triggered to combustion or explosion by a spark.